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Tire shaving to match tread depth --- is it really worth it ?? - NO!

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TBoz-88

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Discussion starter · #1 · (Edited)
Well here's the deal.

I got a 2018 RAV4 LE AWD with 210,000 miles. All four of my Bridgestone DriveGuard run-flat tires (225/65R17) are worn down to around 3 mm tread depth. I just had to replace one of them because it had an irrepairable puncture near the sidewall. So the new cheapo tire has a tread depth of 8 mm and the other three tires have the 3 mm depth.

But here's the thing --- I plan on selling the vehicle in January. So any concerns about damage to the drivetrain because of one tire having 10 mm more diameter than the other tires is alleviated by the fact that I'm dumping the vehicle off to another owner and they won't even know anything bad happened unless they are astute enough to realize one of the tires is new.

I understand some tire shops offer a tread shaving service where they can match tread depth of a new tire to the existing old tires. Is this really worth it in my case? If I can spend $50 and shave down the tire and prevent any major drivetrain damage occurring in the next few weeks, then it's worth it just for my own peace of mind. But can they shave a tire if it's already been driven a couple hundred miles, or do they need to be brand new and unused?

I'm skeptical that any damage would occur if one tire is not even 1/2-inch a greater diameter compared to the other tires. Can anyone debunk this long-held myth about AWD vehicles suffering major catastrophe to the drivetrain because of uneven tire diameters, or is there really truth to it?
 
#2 ·
there is no myth or mystery

if wheels rotate at different rates -all the time- the differential will
get warmer. depending on what is locked, or not locked,
or has a differential involved, or transfer case...whatever is 'fighting' with
the different diameters and hence wheel speeds will see more heat and more wear
where if you had all same tires there would be less heat and less wear

does it matter in the short term nope. but it matters if you disregard this basic
advice over the long term. go off-roading and use lockers or 4wd mode stuff will
be working the drivetrain harder than a different diameter tire will,
but you'd rather have an easy working drivetrain for all your normal pavement driving
 
Discussion starter · #3 ·
crisps said:
there is no myth or mystery

if wheels rotate at different rates -all the time- the differential will
get warmer. depending on what is locked, or not locked,
or has a differential involved, or transfer case...whatever is 'fighting' with
the different diameters and hence wheel speeds will see more heat and more wear
where if you had all same tires there would be less heat and less wear

does it matter in the short term nope. but it matters if you disregard this basic
advice over the long term. go off-roading and use lockers or 4wd mode stuff will
be working the drivetrain harder than a different diameter tire will,
but you'd rather have an easy working drivetrain for all your normal pavement driving
Click to expand...
Well that's really the core question --- how much diameter difference does there have to be to induce drivetrain damage, and how quickly does it happen?

I would think just with normal manufacturing standards and quality control, that new tires coming off the assembly line are probably varying by 1-3 mm in diameter. Then a few PSI of tire pressure difference could also affect diameter by 1 or 2 mm. Plus, just the normal bends of the road causing one or more tires to rotate a little less or more as the vehicle goes through a curve. Add all this up and it seems like AWD design engineers already know about these variances and build in a tolerance to it, up to a certain degree. I just wish I knew what that degree is. I'm hoping that a 1/2-inch wider tire is within those tolerances.
 
Discussion starter · #5 · (Edited)
Uncle Rico said:
Have a read of this thread and info provided by @Dr. Dyno

Click to expand...
Thanks for the link. According to Dr. Dyno's manual references, it looks like I can rest easy about this new tire. The total difference in tire diameter between the new left front tire and worn-down right front tire is about 10mm total, which on a 28-inch diameter tire works out to be around 1% difference in diameter. So still well within the 3% margin of safety.

But I noticed Dyno keeps referencing 3% tolerance between front and rear diameters? Should I move this new tire to the rear so that my AWD can disengage if it senses too much a discrepancy in tire diameters (even left vs. right)? I doubt this would happen since 1% is not close to 3%. I have no problem having my RAV operate in front-drive only mode since I'm selling it within a month and the winters where I live don't dump a lot of snow on the roads.
 
#6 ·
I would put it on the right front (if it is not a directional tread) and drop its pressure a couple of psi. If you were really picky you could measure the effective rolling radius and fine tune the exact psi difference necessary to zero out differential actuation. Remember what will be the case when you run the emergency dummy spare:rolleyes:
 
#9 ·
RD180 said:
Left or right makes no difference - both Rav4.4 differential are open type and do not lock. The only lock is between two front wheels and two rear wheels once AWD is engaged.
Click to expand...
I recommended RF position for a slightly larger dia. tire due to the general slope of roads, lower on the curb side. Those driving on the wrong side of the road:LOL: put it on the left front! Pretty inconsequential anyway.
 
Discussion starter · #10 ·
crofter said:
I recommended RF position for a slightly larger dia. tire due to the general slope of roads, lower on the curb side. Those driving on the wrong side of the road:LOL: put it on the left front! Pretty inconsequential anyway.
Click to expand...
Technically you're right --- but shouldn't this be destroying millions of AWD vehicles on the road every year? According to "experts", having your right tires rotating a couple more times per 50 miles compared to the left is gonna cause many thousands in damage to these delicated and sophisticated drivetrains :p
 
#13 ·
Has anyone consulted w a tire site, like Tirerack? Surely their experts should know about shaving tires as a solution to the new vs old tire for AWD vehicles. A phone call to their technical guys may solve the potential problem. Just askin
 
#14 ·
Do you really expect a commercial company give you an unbiased answer at expense of their profits and potential liability? I am more than sure they will advise you just to buy 4 new tires. Not to mention that in no way they would be an expert in how a a particular AWD system works on a particular trim of particular year of particular model.
 
#15 ·
RD180 Do you really expect that there are experts here on this forum on this specialized topic? Yes there are some pretty knowledgeable folks here, but a discussion w a company that specializes in tires is the logical place to go....
And...has anyone consulted with Toyota on the topic.?
The original poster of this thread has tires that are 3mm.... the very minimum for a safe tire.... and wants to get rid of the car in a few weeks......what would you do RD180?
 
#16 ·
I would buy a used tire worn to the same amount on eBay - ideally from same brand and model as the rest. Actually I have done it many times before - and not even because of AWD - just to have even wear on all four and also save a on the cost. Once all four wear out (at the same time) I buy a brand new set.

I also bought Rav4 hybrid iAWD and now I do not have to worry about this "matching tire size" nonsense (two open differentials and no mechanical link front-to-rear on a hybrid).
 
#17 ·
RD180 said:
Do you really expect a commercial company give you an unbiased answer at expense of their profits and potential liability? I am more than sure they will advise you just to buy 4 new tires. Not to mention that in no way they would be an expert in how a a particular AWD system works on a particular trim of particular year of particular model.
Click to expand...
Exactly! Their job #1 is to sell tires, and do it whatever way it takes, scare tactics and "expert" BS included.
 
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#18 ·
RD180 said:
I would buy a used tire worn to the same amount on eBay - ideally from same brand and model as the rest.
Click to expand...
That's what I did, an had to do, when I owned a Forester years ago. All to avoid having to buy 4 new tires or chance burning up the dumb viscous coupler in the driveshaft.

Notwithstanding the fact that Toyota now stupidly calls it a "viscous coupler," and Hybrids and Driveline Disconnect models excluded, 4WD/AWD 4.3-on RAV4s have a much smarter variable electro-coupler at the rear differential. It's electronically operated by the 4WD ECU which engages the rear drive on every take-off until it deems it not necessary at which point it gradually disengages.

It however will not engage if it detects 3% or more difference between front and rear wheels. So not being deliberately stupid with tires to keep the 4/AWD working makes sense, but this tire shaving or tread matching is not necessary to protect the drive system.
 
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#20 ·
That's pretty much what I'd expect from ANY company selling or shaving tires, stay on the money making side.
 
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